Inactivation of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by N-cyclohexyl-N'-(4-dimethylamino-alpha-naphthyl)carbodiimide.

The synthesis and characterisation of N-cyclohexyl-N'-(4-dimethylamino-alpha-naphthyl)carbodiimide (NCD-4) is described. Only the N-acetylurea and urea corresponding to NCD-4 are appreciably fluorescent: the O-phenylisourea and S-ethylisothiourea derivatives have negligible fluorescence. NCD-4 inhibits the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum irreversibly: Ca2+ protects against inhibition. Covalent incorporation of NCD-4 occurs into the Ca2+-protected sites, with a stoichiometry of approximately 1 mole/mole of ATPase. The modified enzyme has fluorescence emission properties similar to those of NCD-4 N-acetylurea in a relatively hydrophobic environment: it is concluded that NCD-4 has modified a carboxylate group (s) located in or near the Ca2+-binding sites of the ATPase.